Gears are commonly used when machines fail to meet load torque or speed requirements. Mechanical gearboxes are widely used to match the prime-movers' operating speeds to the requirements of their loads, both for increasing the rotational speed (e.g., wind turbine generators) or decreasing the speed (e.g., electric-ship propulsion). It is usually more cost and weight effective to employ a high-speed electrical machine together with a gearbox to transform speed and torque than to build a machine to operate at a desired speed. Although high torque densities can be achieved with mechanical gearboxes, there are many concerns about their operation such as acoustic noise, mechanical vibration, need for lubrication, reliability concerns, and maintenance requirements.
One solution to some of the above issues is to employ a gearbox using magnetic gearing in which the rotational forces, or torques, are transmitted across an airgap through the interaction of magnetic fields [1]. Magnetic gears avoid problems associated with traditional mechanical gears. Magnetic gears do not have contacting surfaces, or engaging teeth, and do not require transmission fluids. A further benefit is that magnetic gears have concentric rotating shafts and forces within them tend to be more balanced, relieving the stress on the bearings and thus allowing for significant improvements in bearing longevity. The magnetic gear system is therefore robust and highly reliable. Magnetic gears also eliminate friction losses due to contacting teeth and are highly efficient.
Magnetic gears transmit torque without contact by different magnetic poles through flux modulating pieces. Since the torque transmission occurs without any contact, magnetic gears have high efficiency, low noise, less vibrations and improved reliability. Furthermore, for higher power ratings, magnetic gears are smaller and lighter than mechanical gears and have inherent overload protection. In case of overload, the magnetic gear slips and when the fault is cleared, it reengages safely. Magnetic gears significantly reduce harmful drivetrain pulsations, which cause misalignment/vibration of shafts. Torque densities, typically in the range 40-80 kNm/m, can be achieved with high efficiency [1-9].
Low-speed high-torque electrical machines have many applications. In order to have low-speed and high-torque from an electrical machine, the machine can be designed to operate at a desired speed or the machine output can be geared to the desired speed. Satisfying the load requirements needs to be done with efficiency, low cost, minimal size and simplicity. Since the machine size is directly proportional with the torque, high torque machine are generally large. Direct drive machines at high torques are often not feasible due to size and mass [14-17].
A magnetic gear integrated with a permanent magnet (PM) motor has been developed for a number of applications. Due to its high torque density, PM machines were integrated with magnetic gears and resulted in a high-torque low-speed drive which eliminates the mechanical gearbox with all of its related concerns. It has high reliability, low maintenance, low cooling requirements, inherent torque overload protection and low NVH (Noise, Vibration, and Harshness) [3-9]. PM machines integrated with magnetic gears have been used in many applications such as electric vehicle traction, wind turbines, ship propulsion systems, aerospace actuation, and industrial applications that require a high-torque, low-speed drive [1-9, 14, 16, 17].
There are a number of applications where high torque, low speed characteristics are required but the industrial system to be driven is well suited to an induction machine drive. Examples may include systems having a long cable or an isolation transformer between the drive and motor, or systems which exhibit significant backlash, windup or potential for jamming. In these cases, PM systems are not suitable and induction machines with integrated magnetic gear would be a better fit.
Although PM machines can be made with very high torque densities and high pole numbers (for low speed operation), induction machines offer simplified open loop control or simple ‘sensorless’ closed loop control, lower cost, can be used with systems with long cables requiring isolation or filtering, and are also suitable for systems with significant backlash, windup or potential to jam.
However, the combination of induction machines with magnetic gears is not straightforward. PM machines and magnetic gears are both synchronous. Induction machines are asynchronous and therefore not easily combinable with synchronous magnetic gears.
One example of a PM machine with magnetic gears is disclosed in PCT Publication WO2011/144895 (Large magnetically geared machines). This application discusses a PM machine integrated with magnetic gear, integrating two synchronous machines. The application provides an electrical machine comprising a first rotor, wherein the first rotor includes a support structure, a second rotor, and a stator. The first rotor, second rotor, and stator are arranged concentrically about a shaft, and at least one of the second rotor and the stator accommodates the support structure. It is designed for use in energy generation or propulsion. This system does not incorporate an induction machine with integrated magnetic gearing.
A further example of a PM machine with magnetic gears is disclosed in U.S. Pat. No. 8,968,042 (Electric marine propulsion device with integral magnetic gearing). This patent discloses a propulsion device comprising an electrical machine with integral magnetic gearing, which comprises three members, namely a first or inner rotor comprising a first plurality of permanent magnets, a second rotor in the form of a plurality of ferromagnetic pole pieces, and a stator which is associated with a plurality of 3-phase windings and to the periphery of which a plurality of second permanent magnets are fixed. This patent uses a PM machine. The patent provides a list of machines that may be used in the integration with magnetic gears but it doesn't provide any disclosure of how to combine asynchronous machines with the synchronous magnetic gears.
Another PM machine system with integrated magnetic gears is disclosed in US Publication No. 2011/0012458 (Magnetic drive systems). This application discloses a PM machine integrated with magnetic gear. It has one set of PMs and the windings are put on the stator combining the two stators with each other, resulting in a one stator configuration. The main focus of this prior application is the design and support of ferromagnetic pole pieces within the system. Again, this system integrates two synchronous machines.
A further example of a PM machine with integrated magnetic gears is disclosed in U.S. Pat. No. 8,358,044 (Electric machine apparatus with integrated, high torque density magnetic gearing). This patent shows a flux modulated PM machine with two stators and one rotor. It shows two configurations but again, it integrates two synchronous machines.
An example of a hybrid induction machine with magnetic gears is shown in Mezani et al., “Magnetically Geared Induction Machines”, in Magnetics, IEEE Transactions, vol 51, no. 11, pp 4-8, June 2015. This paper describes a method of coupling a wound rotor induction machine to a magnetic gear to achieve a high torque density drive system. It uses 3-phase windings to increase the torque transmission capabilities of the system. This allows more operational flexibility compared to magnetically geared PM machines. It also uses ferromagnetic pieces in designing the magnetic gear and a multi-phase winding arrangement. Mezani uses a design which incorporates one stator and two rotors. The arrangement described in this paper requires additional direct current (DC) boost windings and additional three phase rotating diode rectifier. This means that the system is not a fully integrated induction machine with integrated magnetic gears, but is a hybrid induction and synchronous machine with magnetic gear. Finally, the design does not allow for optimization of the parameters in the system. These parameters affect the efficiency and operation of the integrated magnetic gear, and also torque density values.
Presently, there are limited options for low speed drives that use induction machines without using mechanical gearboxes, which can be a major point of failure, for example, in Type-3 wind turbines [10]. It would be advantageous to develop an induction machine that is fully integrated with internal magnetic gears for the ability to have a high torque low speed machine.